1. Field of the Invention
The present invention relates to a wireless LAN orthogonal frequency division multiplexing (OFDM) system, and more particularly, to a fast Fourier transform device of a wireless LAN OFDM system capable of performing a fast Fourier transform (FFT) and an inverse fast Fourier transform (IFFT).
2. Description of the Related Art
Nowadays, a wireless communication system enables a mobile multimedia service of a high quality by a continual technique development.
However, the wireless communication system transmits and receives data with a high speed, so that inter-symbol interference (ISI) is generated. According to this, in order to reduce said inter-symbol interference, an orthogonal frequency division multiplexing (OFDM), a modulation technique of the fourth generation is widely being used. The OFDM is performed for parallel data of a low speed, so that a fast equalization is not required, a usable bandwidth can be completely used, and a multi-path fading and impulse noise can be reduced.
The OFDM was mainly used in a military communication at the early days, and now it is widely being used in a digital video broadcasting for terrestrial (DVB-T), a wireless LAN, and etc. The DVB-T, a distant communication method, has a greater delay spread than a wireless LAN, a near communication method, so that a guard interval (GI), a region for preventing the delay spread has to become larger. Therefore, the DVB-T is realized as a fast Fourier transform (FFT) supporting both 2048 points and 8192 points.
On the other hand, the wireless LAN, a near communication method, has a less delay spread than the DVB-T, thereby being realized as an FFT for supporting 64 points.
FIG. 1 is a view showing a transmission block diagram of a general wireless LAN OFDM system;
As shown in FIG. 1, the wireless LAN OFDM transmission system comprises: a forward error control (FEC) coder 101 for performing a convolution encoding by receiving a signal generated from a media access control (MAC) layer; an interleaving/mapping unit 102 for performing an interleaving in order to prevent a burst error due to a fading of a signal outputted from the FEC coder 101 in a wireless section and performing a mapping so as to fit to a transmission speed of each signal; an inverse fast Fourier transform (IFFT) unit 103 for performing a multi carrier modulation (MCM) based on the mapped signal; a GI insertion unit 104 for inserting a GI to a signal outputted from the IFFT unit 103; a symbol wave shaping unit 105 for pulse-shaping filtering a signal outputted from the GI insertion unit 104; an IQ modulation unit 106 for modulating a signal outputted from the symbol wave shaping unit 105; a multiplier 107 for up-converting the modulated signal (converting into a wireless frequency signal); and a high power amplifier (HPA) 108 for amplifying the up-converted signal.
Transmission processes of the wireless LAN OFDM system will be explained in more detail as follows.
First, the FEC coder 101 receives a signal generated from a MAC layer thus to perform a convolution encoding, and thereby outputs. The interleaving/mapping unit 102 performs an interleaving in order to prevent a burst error of a signal outputted from the FEC coder 101, and performs a mapping so as to fit to a transmission speed of each signal by digital modulation methods such as a binary phase shift keying (BPSK), a quadrature phase shift keying (QPSK), 16 quadrature amplitude modulation (QAM), and 64 QAM.
The IFFT unit 103 performs an MCM which is a core technique of the OFDM on the basis of a signal mapped in the interleaving/mapping unit 102. Herein, the MCM converts serial data of a high speed into parallel data of a low speed and thus employs frequency division multiplexing (FDM), thereby transmitting the parallel data of a low speed without an influence of a multi-pass.
The GI insertion unit 104 inserts a GI corresponding to approximately a quarter of a length of an-FFT into a signal outputted from the IFFT unit 103. Also, the signal into which the GI is inserted is used in a synchronization algorithm such as a carrier recovery or a symbol timing recovery.
The symbol wave shaping unit 105 performs a pulse-shaping for the signal into which the GI is inserted in order to limit a transmission signal bandwidth of the signal, and the IQ modulation unit 106 modulates the pulse-shaped signal. The multiplier 107 outputs the modulated signal as a form which is finally up-converted into a bandwidth of 5.15˜5.35 GHz and 5.725˜5.825 GHz. Also, the high power amplifier (HPA) 108 amplifies the up-converted signal and transmits the amplified signal through an antenna.
FIG. 2 is a view showing a reception block diagram of a general wireless LAN OFDM system.
As shown in FIG. 2, the general wireless LAN OFDM reception system comprises: a low noise amplifier (LNA) 201 for minimizing noise generation of data received through an antenna and thus amplifying; a multiplier 202 for down-converting (converting into a middle frequency signal) the amplified data; an auto gain control (AGC) amplifier 203 for adjusting a gain of the down-converted signal; an IQ signal detector 204 for converting the signal of which gain was controlled into a baseband signal; an auto frequency control (AFC) clock recovery 205 for generating a clock to be used in the IQ signal detector; a GI removing unit 206 for removing a GI which has been inserted into a signal converted by the signal detector 204; an FFT unit 207 for performing a fast Fourier transform in order to convert the signal of which GI has been removed into a signal of a frequency region and thus generating a forward error correction (FEC) coded mapping sample; a demapping/deinterleaving unit 208 for deinterleaving and trellis-decoding the generated mapping sample; and an FEC decoder 209 for recovering a signal outputted from the demapping/deinterleaving unit 208 into data generated from the MAC of the transmission system and thus transmitting to an MAC of a receiving end.
As aforementioned, in case that a physical layer is formed in the wireless LAN OFDM system, the wireless LAN OFDM system has to constitute a hardware which performs the FFT/IFFT.
However, in the conventional wireless LAN OFDM system, the FFT/IFFT can not be shared in one hardware, thereby increasing a production cost and occupying much installation space.